1. Field of the Invention
This invention relates to a group III-V nitride-based semiconductor substrate and, in particular, to a group III-V nitride-based semiconductor substrate that can prevent problems such as an ohmic failure of an electrode, and a characteristic failure and a reliability failure of a device. Also, this invention relates to a group III-V nitride-based light emitting device produced using the semiconductor substrate.
2. Description of the Related Art
Group III-V nitride-based semiconductor materials such as gallium nitride (GaN), indium gallium nitride (InGaN) and gallium aluminum nitride (GaAlN) have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be applied to a short-wavelength light emitting device. Further, they have a high saturation drift speed of electron and can use two-dimensional electron gas obtained by hetero junction. Therefore, they are also expected to be applied to an electronic device.
In fabricating a semiconductor device, so-called homoepitaxial growth is generally conducted where a substrate with the same lattice constant and linear expansion coefficient as a crystal to be epitaxially grown thereon is used as a base substrate. For example, aGaAs single crystal substrate is used as a substrate for the epitaxial growth of GaAs and AlGaAs. However, a group III-V nitride-based semiconductor substrate sufficient in size and characteristics has not been developed for group III-V nitride-based semiconductor crystals such as GaN. Therefore, a single crystal sapphire has been used as a substrate for GaN growth, and GaN has been hetero-epitaxially grown thereon by a vapor phase growth method such as MOVPE (metal-organic vapor phase epitaxy), MBE (molecular beam epitaxy), HVPE (hydride vapor phase epitaxy) etc.
However, since the sapphire substrate has a lattice constant different from GaN, a single crystal GaN film cannot be grown directly on the sapphire substrate. Thus, a GaN growth method is devised that uses a low temperature buffer layer technique which has been developed for hetero-growing Si etc. on the sapphire substrate. The technique is conducted such that the buffer layer of AlN or GaN is formed on the sapphire substrate at a low temperature of about 500° C., and then the GaN is grown thereon while reducing lattice distortions by the low temperature growth buffer layer.
By using the low temperature growth nitride layer as the buffer layer, the single crystal epitaxial growth of GaN can be achieved on the sapphire substrate. However, in growing the low temperature buffer layer, since the optimum range of condition settings for growth temperature and growth thickness is narrow, it is difficult to achieve good reproducibility.
Therefore, a measure may be taken that, instead of continuously growing the entire epitaxial device structure on the sapphire substrate, so-called GaN templates with only the GaN single layer grown on the sapphire substrate are produced in lump or outsourced, and then the remaining device structure is epitaxially grown on the GaN template in separate process.
However, in the templates with only the GaN grown on the sapphire substrate, even when using the low temperature buffer layer technique, the lattice mismatch between the substrate and the GaN crystal is beyond any controls so that the obtained GaN must have a dislocation density as high as 109 to 1010 cm−2. This defect becomes an obstacle to fabrication of a GaN-based device, especially LD and ultraviolet LED. Thus, the GaN templates are used exclusively for a visible-light LED and an electronic device whose device characteristics are not so seriously affected by the dislocation.
On the other hand, for LD and ultraviolet LED devices to require an epitaxial growth layer with a very low dislocation density, a method is proposed in which a substrate formed of only a GaN material is used as a crystal growth substrate and a semiconductor multilayer film composing its device part is formed on the substrate. Herein, such a crystal growth GaN substrate is referred to as “GaN freestanding substrate”.
The GaN freestanding substrate is generally prepared such that a thick GaN layer with a reduced dislocation density is epitaxially grown on a hetero-substrate such as a sapphire substrate, and then the GaN layer is separated from the base hetero-substrate after growth thereof to obtain the GaN freestanding substrate.
For example, a method is proposed that, by using a so-called ELO (epitaxial lateral overgrowth) technique, a mask with openings is formed on a base substrate of sapphire etc., a GaN layer with a low dislocation density is laterally grown through the openings, and the base sapphire substrate is removed by etching etc. to obtain the GaN freestanding substrate (See, e.g., JP-A-11-251253).
Another method is also proposed that, by using a VAS (void-assisted separation) method, voids (i.e., a network of TiN thin film) are formed on a base substrate of sapphire etc., a GaN layer is grown thereon through the voids, and the GaN layer is separated at the interface between the base substrate and the GaN layer, so that both separation of the GaN substrate and reduction of the dislocation density can be achieved simultaneously (See, e.g., Y. Oshima et al., “Preparation of Freestanding GaN Wafers by Hydride Vapor Phase Epitaxy with Void-Assisted Separation.”, Jpn. J. Appl. Phys. vol. 42 (2003) pp. L1-L3).
However, it is difficult to grow, directly on the “as grown” GaN substrate obtained by the above methods, an epitaxial layer for fabrication of a device since morphology such as pits and hillocks usually appears on the surface of the substrate. Therefore, the GaN substrate is generally used after the surface of the substrate is polished and mirror-finished.
In general, a bulk semiconductor crystal may have a multilayer structure that layers with a different impurity concentration are periodically stacked in the crystal growth direction (i.e., in the thickness direction of the substrate). It is assumed that the multilayer structure is caused by rotating the crystal during the crystal growth, whereby the crystal is forced to periodically pass through a region with a temperature gradient or a region with a different concentration in material or dopant, so that the crystal layers with the different impurity concentration are stacked. The behavior of the multilayer structure can be also reflected by that the impurity concentration has a constant period of amplitude to be determined by number of rotations and growth rate of the crystal, when measuring a distribution of the impurity concentration in the thickness direction of the substrate by means of SIMS etc.
Further, when the grown crystal is cut parallel to the growth direction and, subsequently, a certain kind of etching is conducted to the cut surface or an excitation light is irradiated thereto to observe its luminescence behavior, a constant period of stripe pattern can be clearly observed that is formed parallel to “crystal growth interface” which means a virtual crystal growth interface which must have existed at its observation position just when the crystal is growing at the observation position. The stripe observed is called “striation” or “growth striation”. In the crystal, the shape of the crystal growth interface at the time when a specific region of the crystal grows can be left as unevenness in impurity concentration, i.e., striation line, where the history of the crystal growth interface can be checked by pursuing the transition of the line. Since the period of the stripe pattern is sometimes longer than that of the distribution of the impurity concentration, both are not always equal to each other. However, the periodical appearance of the stripe pattern reflects that a periodical change of the impurity concentration exists in the thickness direction of the substrate.
Such a striation is generally formed in the thickness direction of a compound semiconductor substrate of GaAs or InP. However, since the substrate is cut out of a bulk crystal grown from a melt, it is assumed that the striation in the thickness direction of the substrate is relatively small in difference of impurity concentration and, therefore, it does not adversely affect its device fabrication. In other words, the significant difference of the impurity concentration as described earlier is caused by that, in rotating the crystal during the crystal growth, the crystal is forced to periodically pass through a region with a temperature gradient or a region with a different concentration in material or dopant, so that such a difference can occur in the concentration of impurity to be introduced into the crystal. By contrast, in case of the crystal growth of GaAs or InP, since the crystal growth interface always contacts the common surface of the melt, it can be grown under circumstances that are significantly smaller in temperature gradient and dopant concentration gradient than GaN crystal described later.
On the other hand, since the GaN substrate is produced by the vapor phase growth method as mentioned earlier, it often grows periodically passing through the circumstances with a steep temperature gradient, different than the other compound semiconductor crystal grown from the melt. Further, the crystal growth rate at a passage position of the crystal depends on the concentration of a raw material gas existing at the position, and the concentration of an impurity to be introduced at the position depends on the concentration of a dopant gas existing at the position. However, since both of the concentrations are determined by the flow distribution of the raw material gas in the reactor, variation in both concentrations is likely to be significantly greater than the crystal grown from the melt. In spite of this, even when the raw material gas flows disproportionately in the reactor, the crystal may be apparently formed with uniformed thickness and impurity concentration by averaging based on the rotation of the crystal substrate. However, since the crystal actually grows as it passes through alternately a significantly high growth rate and a significantly low growth rate, on close investigation a structure is often found in which layers with a very large difference in impurity concentration are stacked alternately.
The carrier concentration of a semiconductor substrate is generally measured by the van der Pauw method or eddy-current measuring method. However, since these methods are only adapted to measure a carrier concentration averaged in its bulk, they are not adapted to measure a periodical distribution of the impurity concentration in the thickness direction of the substrate even when it exists.
Thus, as described above, to have a striation with a very large difference in impurity concentration can be equivalent to stack alternately a high carrier concentration layer and a low carrier concentration layer. Therefore, even when the average carrier concentration of a substrate is apparently high, problems may arise that an ohmic contact is not achieved between the substrate and an electrode attached thereto, and a device adapted to flow current perpendicularly to the substrate is subjected to an increase in operating voltage, an increase in heat generation and a reduction in reliability.